The present invention relates to cellulosic fuels in the form of pellets bound with a plastic binder.
Due to diminishing quantities of coal, petroleum, and natural gas products, attention is being directed to other energy sources. One source which is receiving considerable attention is biomass materials such as wood, bagasse, their byproducts, and agricultural residues.
Use of compressed wood waste, pellitized or briquetted, for example, as a fuel source has achieved only limited acceptance to data. One reason for this is the relatively low realized heating value of compressed waste. Compressed wood wastes also have a slow burning rate. Some of these wastes have exhibited incomplete burnout, resulting in the formation of carbonaceous residues and low combustion efficiency. In addition, compressed wood can be hard to ignite. Another problem is in the fraility of compressed wood which leads to special handling to avoid crumbling, the creation of fines and dust, and the avoidance of weathering.
To overcome the crumbling and weathering problems, inorganic binders, such as cement and silicate of soda, and organic binders, such as tar, pitch, rosin, glues, waxes and fibers, have been included in the pellets. However, no binder has been found which completely solves the above problems, and which also is inexpensive and does not reduce the heating value of the wood.
Attempts have been made to use the self-binding characteristics produced from lignin in various species of wood to avoid the crumbling problem. This can be done with some species of wood, but not all species, by heating the wood above the minimum plastic temperature for lignin of 163.degree. C. However, such a wood pellet still does not have a high mechanical strength. Furthermore, such high temperatures can severely limit the operating life of the pelletizing equipment, drive high BTU volatile components from the wood, and lose some energy because of the requirement of heating.
Specific examples of prior art approaches include the disclosures of British patent specifications No. 901,789 to Stamicarbon, Japanese patent application No. 46-10282 of Masoyoshi, U.S. Pat. No. 3,947,255 to Hartman et al, U.S. Pat. No. 3,843,336 to Messman, and U.S. Pat. No. 4,015,951 to Gunnerman. Stamicarbon discloses a fuel briquette of coal particles and a binder of an olefinically unsaturated hydrocarbon. The fuel particles are smaller than 3 mm. The bonding of the binder to the coal particles is by melting the binder to distribute it throughout the coal, dissolving the binder to effect absorption on the coal, or effecting a skin of binder on the coal particles with a tar oil distillate and hot compression. Stamicarbon uses only a modest amount of plastic, one to two percent, and discloses a moisture content of from five to eight percent. The Masayoshi patent application discloses a fuel of 9 to 66 percent thermoplastic in a balance of wood meal or chips. Masayoski melts the thermoplastic to combine it with the wood meal or chips. Hartman uses from 2.5 to 40 percent plastic as a binder for bark. The plastic is melted to do its job. The moisture content of Hartman's bark is less than 7%. Messman discloses an artificial fireplace log of thermosetting resin, sawdust, wax and fuel oil. Messman effects a sheath of plastic on the outside of an extrudate, the log, and uses a high percentage of plastic. Gunnerman uses fibrous material, he examples wood, with a moisture content of from 16 to 28%.He compresses the material with a die such that the temperature of the pellet as it leaves the die is from 325.degree. to 350.degree. F. (163.degree. to 177.degree. C.), and then dries the pellets. He states that pellets made by his process are held together by interlocking of broomed out, fibrous particles, and possibly heat softened lignin. The size of the individual particles is not more than 85% of the minimum dimension of the pellets.
In U.S. application Ser. No. 943,393 of Ian Fraser Johnston, a pellet of cellulosic material and thermoplastic is disclosed. The Johnston pellet has from 1 to 10% thermoplastic in a balance of cellulosic material. That material contains from 5 to 15% moisture. Both the plastic and cellulosic material are particles small enough to pass through a 5 mesh screen. Johnston discloses a unique bonding between the cellulosic constituent and the plastic. Instead of melting or dissolving the plastic, he softens it so that upon extrusion it interstitially penetrates the fibers of the cellulosic material to produce a mechanical lock between pieces of the material. Johnston's pellets also burn better than the individual components alone. It is thought that the small plastic particles act as ignition sites which liberate combustible gases that progress from the sites into the cellulosic material. There, the combustible gases burn and enhance the ignition and burning of the cellulosic material. The cellulosic material separates the individual plastic particles and permits their burning independently of one another, avoiding the charring attendant with the burning of larger plastic particles. In connection with the enhanced combustion noted in the Johnston pellet, it is known from the work of others that in normal combustion a lignin constituent of cellulosic materials like wood has a high heat of combustion relative to cellulose, but that lignin tends to pyrolyze to char and not burn completely. The char tends to burn slowly in solid phase combustion by smoldering with low rate of heat release. The products of combustion of the lignin in this combustion domain are high in combustible content. If the heat flux increases, the lignin can burn in flaming combustion, leaving little ash. (See Shafizadeh and Bradbury, Smoldering Combustion of Cellulosic Materials, Journal of Thermal Insulation, Vol. 2 (January, 1979).) The combustion products of the Johnston fuel are very low in carbonaceous ash content relative to a natural lignin containing cellulosic material burned under the same conditions. This has lead to the hypothesis that the particulate plastic creates a combustion environment that completes the combustion of any lignin constituent of the cellulosic material, liberating the considerable heat of combustion of lignin, and leaving very little carbonaceous ash.